I. Field of the Invention
The invention relates to the drive assemblies of vehicles comprising an internal combustion engine, and control methods for such assemblies.
II. Description of Related Art
FIG. 1 represents one conventional exemplary embodiment of such an assembly. The assembly 2 forms a motor vehicle drive assembly. In a manner known per se, this assembly comprises an internal combustion engine 4, such as a petrol motor or a diesel motor. It comprises an alternator-starter formed by a variable reluctance machine 6 of a type known per se. This machine 6 can be mechanically linked to the motor 4 to provide the latter with a torque, particularly in order to start it when it is stopped. This mechanical link of a type known per se carries the reference 8 in FIG. 1. The assembly comprises a control unit 10 comprising conventional computer means enabling it to control the various units of the assembly as will be seen below. The assembly 2 also comprises an inverter 12 by means of which the control unit 10 drives the operation of the electronic machine 6.
The assembly 2 also comprises a position sensor and/or a speed sensor 14 enabling the unit 10 to know a measurement of the speed of the machine 6 at each instant.
FIG. 2 represents a conventional inverter topology for controlling a motor phase. In this FIG. 2, VDC represents the DC voltage powering the inverter, I1 and I2 two electric switches that can be switched open and closed and D1 and D2 are two diodes.
A control method for such a vehicle drive assembly (comprising, as has been seen, a reluctance machine associated with an inverter) is normally used to control the torque delivered by this drive assembly.
Even more specifically, such a control method of a vehicle drive assembly is designed, for a given power supply voltage, and for a given inverter/reluctance machine assembly, to deliver the greatest possible torque (both in motor mode and in generator mode) over the operating range of the machine, and this mainly at high speeds.
The effective aim is to achieve the best cost/weight/volume trade-off for the machine-inverter drive assembly.
FIGS. 3 and 4 illustrate the shape of the phase currents and voltages as represented in FIG. 2, respectively at low speed and at high speed, according to the electrical angle of the rotor (this angle being zero when the tooth of the rotor is in conjunction with the tooth of the stator).
It is known that a variable reluctance machine such as the machine 6 in FIG. 1 is essentially controlled by three variables:                the start of magnetization ON and end of magnetization OFF angles;        and, at low speeds, the peak phase current which is regulated (Ilim).        
The phase of the machine is magnetized in the time interval formed by the conduction angle Θp=OFF-ON.
It will be noted that, to obtain a complete demagnetization, the conduction angle Θp must be less than 180°.
In order to control the variable reluctance machine, the control parameters (ON, Θp, Ilim) at low speed and (ON, Θp) at high speed are used as input parameters for the control unit 10; these parameters being optimized for each torque-speed operating point.
As illustrated in FIG. 1, and in a manner known per se, the machine 6 is controlled by the control unit 10 by means of control laws taken from tables that give the ignition (ON) conduction (Θp) and peak current (Ilim) angles according to the speed of the machine obtained from the sensor 14. These quantities are also determined according to the torque to be delivered by the link 8. If necessary, it is also possible to take account of the DC power supply voltage VDC of the inverter if the latter is likely to vary, and other parameters such as the temperature of the windings, for example.
In each electrical period, a pointer is defined according to these external parameters (speed, power supply voltage, temperature of the windings, etc.) and the torque set point. This pointer then addresses the angle tables which give the control parameters (ON, Θp, Ilim) at low speed and (ON, Θp) at high speed.
The torque delivered by the machine 6 at constant speed is proportional to the energy transmitted by a phase of the motor. It is therefore proportional to the surface area of the curves represented in FIGS. 5 and 6 which illustrate the trend of the flux associated with a phase of the machine as a function of the phase current, for a machine operating respectively at high speed and at low speed.
The energy derived from the electromechanical conversion is thus characterized by the surface area (or energy cycle) delimited by the path taken by the phase flux and the phase current during an electrical period. FIGS. 5 and 6 thus represent the energy transmitted in an electrical period for a machine operating respectively at high speed and at low speed.
It can be seen that, at high speed, the energy cycle is very small compared to the quantity of energy that is potentially usable, that is, as delimited by the minimum phase inductance (teeth in opposition), maximum phase inductance (teeth in conjunction), and the maximum allowable phase current Ilim.
At low speed, however, it can be seen that the energy cycle is better used, the path taken delimiting almost all the maximum surface area.
In order to make up for the low efficiency at high speed de facto limiting the torque available on the motor shaft, an operating mode called continuous current mode has been proposed whereby a conduction angle Θp greater than 180° electrical (the fluxing time being greater than the defluxing time) is applied.
Such a conduction angle Θp greater than 180° for operation in continuous current mode can be expressed Θp=ΔΘp+180°, where ΔΘp is called additional conduction angle.
FIGS. 7 and 8 show the benefit of the continuous current mode: for one and the same inverter-machine assembly and one and the same power supply voltage, a significant torque gain can be obtained at high speeds.
FIG. 7 (respectively 8) can be used to compare the maximum torque (respectively power) as a function of the speed that can be obtained with the conventional control law (broken lines) and with the continuous current mode control law (solid lines).
It can be seen that a net torque (respectively power) gain is obtained when the continuous current mode is used.
FIG. 9 shows the trend, under the effect of the application of a conduction angle Θp greater than 180°, of the energy transmitted over several electrical periods at constant rotation speed. The gradual saturation from period to period produces a rise in the energy cycle (i.e. the surface area covered) and therefore in the torque available on the motor shaft.
FIG. 10 shows the gradual increase in the average torque delivered by the machine over each electrical period, relative to the first period, given the same conditions of operation as those of FIG. 9. The discontinuity in the torque increase represented in FIG. 10 originates from the action of a regulation law that acts on the control laws (ignition and blocking angles, and peak current limiting) so as to keep the energy cycle, and therefore the torque, constant.
It should be stated here that the torque gain depends:                on the speed of the machine,        on the very design of the machine,        on the control applied (angles, current limiting, free-wheeling).        
As has been seen above, in particular in light of FIGS. 7 and 8, the continuous current mode is advantageous in that it provides for a significant torque gain at high speeds.
The continuous current mode is, however, an intrinsically unstable mode, in which the stability of the flux and the stability of the current are difficult to control.
Moreover, as has been seen above, when the continuous current mode is used, the average torque increases over several electrical periods before reaching its set point level (unlike a conventional operation, in discontinuous mode, for which the desired average torque is obtained after the first electrical period). In continuous current mode, the response of the system to a desired set point torque is therefore fairly unresponsive.
Document EP 0 534 761 envisages stabilizing an operation in continuous current mode, obtained by means of an increase in the conduction time of the machine beyond half of an electrical period, in order to exploit the torque gain at high speeds offered by this operating mode. This document shows that it is possible to regulate the flux, and therefore indirectly the torque, by varying the conduction angle Θp.
However, this document simply uses control parameters preprogrammed and stored in a mapping table and in no way specifies how the conduction angle Θp is varied. A fortiori, this document does not envisage any dynamic control law of the conduction angle Θp.
Moreover, this document does not in any way broach the issue of the low responsiveness of the system in continuous current mode to reach a set point torque.